Fate and destiny reconsidered - How sensitive are neurons in our brain to quantum effects?

During an existing Fluther debate I ran into this issue. I discovered a question in a different forum

http://www.physicsforums.com/showthread.php?t=249305

where Iamu asked and wrote the following:

How sensitive are neurons and chemical receptors like olfactory receptors to the chemicals that stimulate them? Can a single molecule stimulate a nerve? I ask because I’m curious as to whether or not human thought and behavior is generally sensitive to random quantum effects. If a single photon could stimulate a rod, or a single molecule of a neurotransmitter could cause a neuron to fire, then our behavior might be truly random, because it would depend on truly random quantum events, but if it takes thousands of photons or thousands of molecules of a neurotransmitter, I figure our behavior would probably be governed by more-or-less deterministic classical physics.

There are only 2 answers and here’s an excerpt:

Membrane potential is constantly fluctuating due to random openings and closing of ion channels and the like. Mini synaptic events also occur stochastically probably due to quantum fluctuations (...). Randomness is not the same as free will. I’m not sure anyone has a particularly satisfying answer as to how “free will” gets into a neural system classical or not.

Keep in mind that biological systems are very noisy and often the cell employs a variety of mechanisms to cope with that noise. Even though single photon/molecule sensitivity of sensory neurons would lead to stochastic fluctuation in the signals going to the brain, the brain may have evolved control systems to deal with those fluctuations.

I wonder what our Fluther pundits have to say about this matter. Can physics prove that fate is impossible in our universe (let’s not look at the multiverse implications for the time being)? Computers are equipped with extra redundancy to deal with quantum uncertainty. Does it work the same with with our brains?